1. ELECTRICAL ENGINEERING COLLEGE
GILAN UNIVERSITY
ELECTRICAL ENGINEERING COLLEGE
Device fabrication I:
Layer Deposition
By: Ebad ghorbandust – Moin moshtagh – Keyvan alireza zad

2. Objectives Electrochemical Plating (ECP) Vacuum evaporation
Sputtering methods
Chemical Vapor Deposition (CVD)
Vacuum pumping methods

3. Scales to measure vacuum
Atmospheric pressure at sea level and 0°C
751 Torr
1013 mBar
101,330 Pa
14.7 PSI
29.92 inches of mercury
33.79 feet of water
Torr (USA)
mBar (Europe)
Pa - PSI (Asia)
1 atmosphere = bar
1 atmosphere = torr
1 atmosphere = ×105 Pa
1 torr = mm Hg
1 micron Hg = milliTorr
1 millibar = Pa
1 torr = Pa
1 millibar = Torr

4. Electrochemical Plating (ECP)
Electroplating has emerged as a production copper deposition method due to its low temperature and low cost. Electroplating of copper
has been a mainstay of printed circuit board processing
for decades.
The wafer is suspended in a bath containing copper sulfate (CuSO4) and is connected to an cathode (negative pole).
With the application of a current, the bath components disassociate. The copper plates out on the wafer and hydrogen gas is liberated
at the anode.
One concern is the uniformity of the film across
the wafer. Another concern is buildup of a
bevel at the lip of the opening.

5. Deposition Methods Physical Chemical CVD Sputter deposition
Metallization techniques, like other fabrication processes, have undergone improvements and evolution in response to new circuit requirements and new materials.
Filaments (Thermal)
Electron beam
Vacuum evaporation
Physical
Sputter deposition
Chemical CVD

6. Deposition Methods
The mainstay of metal deposition up to the mid-1970s was vacuum evaporation. The needs of depositing multi-metal systems and alloys, along with the need for better step coverage, led to the introduction of sputtering as the standard deposition technique for VLSI circuit fabrication. Refractory metal use has added the third technique, CVD, to the arsenal of the metallization engineer.

7. Vacuum Evaporation
Vacuum evaporation is used for the deposition of metals on discrete
devices and circuits of lower integration levels.
It is also used for the deposition of gold to the back of a wafer for die
adhesion into a package.
The vacuum is required for a number of reasons. First is a chemical
consideration. If any air (oxygen) molecules were in the chamber when the high-energy aluminum atoms were coating the wafer, they would form aluminum trioxide (Al2O3), a dielectric that, if incorporated into the deposited film, would compromise aluminum’s role as a conductor. A second requirement for vacuum deposition is uniform coating.
Filaments and electron beam sources are the usual methods of evaporation.

8. Vacuum Evaporation

9. Thermal Evaporator Wafers Aluminum Vapor 10-6 Torr To Pump
High Current Source

10. Thermal Evaporator Contamination is not under control
Advantage: to set up
Contamination is not under control
Impossible to deposit composition of metal
Disadvantage
Thick layer is difficult to deposit

11. Electron Beam Evaporator
Wafers
Aluminum Charge
Aluminum Vapor
Electron Beam
10-6 Torr
To Pump
Power Supply

12. Electron Beam Evaporator
Shadowing problem
Rotation in two direction

13. Shadowing problem

14. Sputter deposition
Sputter deposition formulated in 1852 by Sir William Robert Grove.
Sputtering (in general) can deposit any material on any substrate. It is widely used to coat costume jewelry and put optical coatings on lenses and glasses.
Sputtering, like evaporation, takes place in a vacuum. However, it is a physical, not a chemical process and is referred to as physical vapor deposition (PVD).

15. Sputter deposition
Inside the vacuum chamber is a solid slab, called a target, of the desired film material. The target is electrically grounded. Argon gas is introduced into the chamber and is ionized to a positive.
charge

16. Sputter deposition
The positively charged argon atoms are attracted to grounded target and accelerate toward it. During the acceleration, they gain momentum, which is force, and strike the target. At the target, a phenomenon called momentum transfer takes place. Just as a
cue ball transfers its energy to the other balls on a pool table, causing them to scatter, the argon ions strike the slab of film
material, causing its atoms to scatter. The argon atoms “knock off” atoms and molecules from the target into the chamber. This is the sputtering activity.
The sputtered atoms or molecules scatter in the chamber with
some coming to rest on the wafer.

17. Sputtering advantages
conservation of target material composition.
deposition of alloys and dielectrics.
Improved adhesion
Advantages

18. Improve the sputtering
The higher energy of the arriving atoms makes for a better adhesion.
the plasma environment inside the chamber has a scrubbing action of the wafer surface that enhances adhesion.
Adhesion and surface cleanliness can be increased by grounding the wafer holder and sputtering the wafer surface for a brief time prior
to the deposition.
Step coverage is further improved by rotating the wafer holder and by
heating the wafer.
improve step coverage and uniform film formation in deep holes is a
collimated beam.

19. Collimator

20. Chamber pressure
Clean and dry argon (or neon) is required to maintain film composition characteristics, and low moisture is required to prevent unwanted oxidation of the deposited film.
Control of the argon amount entering the chamber is critical due to its effect of raising the pressure in the chamber. The chamber is loaded with the wafers, and the pressure is reduced by pumps to
the 1 × 10⁻⁹ torr range .With the argon and sputtered material in the
chamber, the pressure rises to a level of about 10⁻³ torr.

21. Dark space
After liberating material from the target, the argon ions, the sputtered material, gas atoms, and electrons generated by the sputtering process form a plasma region in front of the target . The plasma region is evident by its purple glow. The plasma region is separated from the target by a darkened region, known as the dark space.

22. sputtering methods
Four sputtering methods are used in semiconductor applications.
■ Diode [direct current (dc)]
■ Diode [radio frequency (RDI)]
■ Triode
■ Magnetron

23. DC Diode sputtering
The target is connected to a negative potential with a positively charged anode present in the chamber. The negatively charged target ejects electrons, which accelerate toward the anode. Along the way , they collide with the argon gas atoms, ionizing them. The positively ionized argon atoms then accelerate to the target, initiating the sputtering process. The ionized argon (+) and the target (–) form a diode.

24. Dark space characteristic
Dark spaces exist just in front and to the sides of the target. Sputtering efficiency is enhanced when the plasma is confined to the region between the target and wafers. This condition is enhanced by placing dark space shields to the sides of the target. The shields prevent target material from being sputtered from the sides material that will never deposit on the wafers.

25. Virtual Leak
Arises from the out gassing of contamination from the chamber walls while the chamber is under vacuum. This condition is called a virtual leak. Besides compromising the pressure level in the chamber ,the contamination can be incorporated into the deposited film.
This latter problem is addressed by placing a small negative bias on the wafer holder. The bias creates ions on the wafer surface and has the effect of dislodging stray out gassed atoms from the growing film . Direct-current diode sputtering is used primarily to deposit metals.

26. RF Sputtering
Improved sputtering is gained by connecting the target to the negative side of a radio-frequency (RF) generator.
Radio-frequency sputtering is necessary to sputter non conducting materials (dielectrics) and is used also for conductors.
Biasing is also used with the
radio-frequency sputtering to
achieve a cleaning effect at the
wafer surface.

27. RF Sputtering
Etching and cleaning are achieved by putting the wafer holder at a different field potential from that of the argon, causing the argon atoms to impinge directly on the wafer. This procedure is called sputter etch, reverse sputter, or ion milling.
Removal of contamination improves electrical contact between the exposed wafer regions and the film and improves adhesion of the film to the rest of the wafer surface.
In diode sputtering, a number of processes occur at or near the wa-fer surface. On impact of the argon atom, a number of electrons are created. These electrons cause heating of the substrate (as high as
350°C), which in turn can cause uneven film deposition. The electrons
also create a radiation environment that can damage sensitive
devices.

28. RF Sputtering
The heating produced with diode sputtering causes a serious problem with the deposition of aluminum. The heating causes residual oxygen in the target and in the chamber to combine with the aluminum to form aluminum oxide. The aluminum oxide is a
dielectric and can compromise the conductive property of the deposited aluminum. More serious, a layer of aluminum oxide can
form on the target surface, and the impinging argon atoms do not (in diode sputtering) have enough energy to break through the layer. In effect, the target becomes sealed, and the sputtering stops. Triode sputtering avoids some of the problems of diode sputtering.

29. Triode sputtering
The electrons necessary to ionize the argon are created by a separate high-current filament. In designs where the filament is
located outside the deposition chamber, the wafers are protected from radiation damage. Films deposited by a triode method are more dense.

30. Magnetron sputtering
Another problem with diode sputtering is the electrons that escape into the chamber and do not contribute to the establishment of the plasma necessary for deposition. The situation is resolved in magnetron sputtering systems.
The magnets capture and confine the electrons to the front of the target. Magnetron systems are more efficient for increased deposition rates.
Another effect is a lower pressure required in the chamber, which
contributes to a cleaner deposited film. Magnetron sputtering leaves a lower target temperature.

31. Chemical Vapor Deposition
Chemical vapor deposition (CVD) forms thin films on the surface of a substrate by thermal decomposition or reaction of gaseous compounds. The desired material is deposited directly from the gas phase onto the surface of the substrate. Polysilicon, silicon dioxide, and silicon nitride are routinely deposited using CVD techniques. In addition, refractory metals such as tungsten (W) can also be deposit-ed using CVD.

32. CVD reactors Atmospheric-Pressure CVD(APCVD) Low-Pressure CVD(LPCVD)
Plasma-Enhanced CVD(PECVD)

33. APCVD
This type of reactor has been used for deposition of the silicon dioxide passivation layer as one of the last steps in IC processing. The reactant gases flow through the center section of the reactor and are contained by nitrogen curtains at the ends. The substrates can be fed continuous-ly through the system, and large-diameter wafers are easily handled. However, high gas-flow rates are required by the atmospheric-pressure reactor.

34. LPCVD
The hot-wall, low-pressure system is commonly used to deposit polysilicon, silicon dioxide and silicon nitride and is referred to as a low-pressure CVD system. The reactant gases are introduced into one end of a three-zone furnace tube and are pumped out the other end. Temperatures range from 300 to 1150 °C, and the pressure is typically 30 to 250 pa. Excellent uniformity can be obtained with LPCVD systems, and several hundred wafers may be processed in a single run.

35. Disadvantage LPCVD
Hot-wall systems have the disadvantage that the deposited film simultaneously coats the inside of the tube. The tube must be periodically cleaned or replaced to minimize problems with particulate matter.
In spite of this problem, hot-wall LPCVD systems are in widespread use throughout the semiconductor industry.

36. Plasma-enhanced CVD
CVD reaction can also take place in a plasma reactor. Formation of the plasma permits the reaction to take place at low temperatures, which is a primary advantage of plasma-enhanced CVD (PECVD) processes. In the parallel plate system, the wafers lie on a grounded aluminum plate, which serves as the bottom electrode for establishing the plasma.
The wafers can be heated up to 400°C using high intensity lamps or resistance heaters. The top electrode is a second aluminum plate placed in close proximity to the wafer surface. Gases are introduced along the outside of the system, flow radially across the wafers, and are pumped through an exhaust in the center. An RF signal is applied to the top plate to establish the plasma.

37. Plasma-enhanced CVD
The capacity of this type of system is limited, and wafer must be loaded manually. A major problem in VLSI fabrication is particulate matter that may fall from the upper plate onto the wafers.

38. Comparison of Typical Thin Film Deposition Technology
process
Material
Uniformity
Impurity
Film
Density
Substrate
Temperature
cost
Thermal
Evaporation
Metal or
low
melting-point materials
Poor
High
50 ~ 100 ºC
Very low
E-beam
Both metal
and
dielectrics
Low
sputtering
Very good
Good
~ 200 ºC
PECVD
Mainly
Dielectrics
200 ~ 300 ºC
Very High
LPCVD
Excellent
600 ~ 1200 ºC

39. Vacuum pumps
LPCVD, ion implantation, evaporation, and sputtering processes all
take place in reduced-pressure (vacuum) chambers. Vacuum chambers provide process conditions free of contaminating gases.
In the deposition processes, the vacuum increases the mean free path of the depositing atoms and molecules, which in turn results in more uniform and controllable deposited films.
Atmosphere
(High)Vacuum
Contamination
(usually water)
Clean surface

40. Vacuum pumps
Pumps are selected and used based on a number of criteria, including
Vacuum range required
Gases to be pumped (lighter gases such as hydrogen are more difficult to pump)
Pumping speed
Overall throughput
Ability to handle impulsive loads (periodic outgassing)
Ability to pump corrosive gases

41. Vacuum pumps
Service and maintenance requirements
Downtime
cost

42. Mechanical pumps
Mechanical oil rotary vacuum pumps are a basic design that traces to the 1640s, when Galileo and Torricelli were investigating the theory that air had weight. They are referred to as blower-type
pumps.
Air is removed from the pump by an eccentric rotating vane in a
cavity. As the vane rotates, it compresses and sweeps out the gas
in front of it in the cavity, simultaneously leaving a reduced-pressure
region behind it. The “pushed” material exits through a valve, while
another valve opens to the cavity from the chamber and allows
material in the chamber to flow into the cavity. As the vane rotates,
more and more material is removed from the cavity, thus reducing
the pressure in the chamber.

43. Mechanical pumps

44. Oil diffusion pumps
The oil diffusion pump was the mainstay of most semiconductor vacuum processes. The pump requires the services of an oil mechanical pump to first reduce the pressure in the chamber to
the 10⁻³-torr level. Either the same or a second mechanical pump is
required at the outlet end. High vacuum, in the 10⁻⁸-torr range (under production conditions), is achieved by a clever momentum
transfer system. A low-vapor-pressure, hydrocarbon-based oil is
heated in the base of the pump, where it rises up a structure called
the stack. At the top of the stack is a series of downward-facing
baffles. The hot oil molecules, which have gained speed and energy from the boiling, exit the stack in a downward direction. Outside the
stack, they collide with gas from the chamber, causing them to be propelled toward the bottom of the pump, where they are removed by a mechanical pump. The oil molecules return to the heated
reservoir.

45. Oil diffusion pumps

46. Two problems with oil diffusion pumps are the migration of oil molecules back up into the chamber and the inability of the pump to handle water vapor from the chamber. Both problems are resolved by the use of a cold trap between the pump and the chamber. A cold trap is similar in design to a liquid source bubbler. The fluid in the trap is liquid nitrogen, which reduces the temperature to –96°C. At this temperature, oil, contaminant and water vapor molecules are
frozen to the inside walls of the trap and do not add to the pressure
in the system.
Oil diffusion pumps

47. Even with cold-trap technology, some processes cannot stand contamination from hydrocarbon oils such as are used in oil diffusion pumps. This situation has led to the use of cryogenic (cryo) pumps. A cryogenic Pump uses the fact that gas molecules will “freeze” out
on cold surfaces. The frost that collects on the insides of refrigerators are examples of cryogenic activity.
Cryogenic pumps

48. Cryogenic pumps
Cryopumps are simpler to operate and maintain, since no cold trap liquids or messy oils are needed. Additionally, cryopumps can handle bursts of outgassing from the process chamber and feature a fast pumping speed.

49. Ion pumps
Another capture-type pump is the ion pump, also known as a sputter
ion pump or a getter pump. An ion pump operates in a manner similar to an ionization section in an ion implanter or sputter machine;
only in this application, the atoms and molecules come from the chamber. A portion of those that drift into the ionization Chamber are
ionized to a positive charge by bombardment with electrons and attracted to a titanium cathode (negative potential). On collision with the titanium, some of the titanium is sputtered away and travels into the pump. The titanium atoms are chemically active enough to com-bine with other gases in the pump, which also accumulate on the
pump walls.

50. Ion pumps
Again, material is removed from the chamber which reduces the pressure. Ion pumps are capable of pressures down to 10⁻¹¹ torr,
which is the ultra-high vacuum range.

51. Turbomolecular pumps
Turbomolecular pumps are similar in design to a jet turbine engine. A
series of blades with openings are mounted and rotated at very high
speeds (24,000 to 36,000 rpm) on a central shaft. Gas molecules from the chamber encounter the first blade and gain momentum from the collision with the rotating blade. The momentum direction is
downward to the next blade, where the same thing happens. The net
result is a removal of gas from the chamber. The use of a momentum
transfer makes the pumping principle the same as an oil diffusion
pump. 51

52. Turbomolecular pumps
Major advantages of turbomolecular pumps are a lack of back-streaming from oils, no need to recharge, high reliability, and pressure
reduction into the high vacuum range.